The fact that clinical pain frequently occurs in muscle constitutes a compelling reason for increasing our knowledge of muscle pain and its impact on motor function. When pain is present, the requirements of maintaining posture or the functions of the daily life are such that the activity has often to be performed regardless of pain. However, despite the fact that it is so common, we have very limited understanding of the impact of musculoskeletal pain on motor function and on jaw motor function in particular. Better knowledge will lay a rational foundation for understanding mechanisms underlying clinical craniofacial muscle pain. Our limited knowledge of the effect of pain on motor performance is due in large part to the lack of reliable experimental models for muscle pain. We propose to apply a tested feedback control system to maintain muscle pain at a constant level. This model is based on the computer-controlled infusion of an algesic substance into muscle and has proved to be a reliable way of inducing pain of predictable duration and intensity at a specific site. A control system that utilizes (1) the subject's initial response to a standard stimulus to program the subsequent infusion and (2) feedback from the subject was developed to maintain the level of perceived pain intensity constant over time. We will use healthy subjects in repeated measures design experiments to quantify several aspects of motor function before experimental pain induction, in pain, in imagined pain, and after pain has subsided. Imagined pain of similar intensity will be used for controlling the nonspecific affective reaction. In the case of each specific aim, we will test the null hypothesis (H-O) that no change in motor function occurs in either pain or imagined pain. Specifically, we will investigate the effect of pain on the postural activity of masticatory muscles, on static jaw position, and on standardized dynamic tasks. These studies will be complemented by a series of projects to investigate the effect of pain on the length and tension threshold, the minimum sustainable discharge rate, and task-specific firing patterns of motor units.